Remove bundled rebar to fix RB14 build error.

By bundling a rebar build we tie the project to a limited subset of Travis'
available erlang versions. No good reason to do this unless we have to.

This commit will fix the build error here:
https://travis-ci.org/rackerlabs/gen_script/builds/8558506

Signed-off-by: Brian L. Troutwine <brian.troutwine@rackspace.com>